Add a configure flag for enabling -Werror on the command line
while building as requested by Lang.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@161253 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/autoconf/configure.ac b/autoconf/configure.ac
index 413a1ee..4d3a14e 100644
--- a/autoconf/configure.ac
+++ b/autoconf/configure.ac
@@ -502,6 +502,16 @@
   AC_SUBST(DISABLE_ASSERTIONS,[[DISABLE_ASSERTIONS=1]])
 fi
 
+dnl --enable-werror : check whether we want Werror on by default
+AC_ARG_ENABLE(werror,AS_HELP_STRING(
+  --enable-werror,[Compile with -Werror enabled (default is NO)]),, enableval="no")
+case "$enableval" in
+  yes) AC_SUBST(ENABLE_WERROR,[1]) ;;
+  no)  AC_SUBST(ENABLE_WERROR,[0]) ;;
+  default) AC_SUBST(ENABLE_WERROR,[0]);;
+  *) AC_MSG_ERROR([Invalid setting for --enable-werror. Use "yes" or "no"]) ;;
+esac
+
 dnl --enable-expensive-checks : check whether they want to turn on expensive debug checks:
 AC_ARG_ENABLE(expensive-checks,AS_HELP_STRING(
   --enable-expensive-checks,[Compile with expensive debug checks enabled (default is NO)]),, enableval="no")